1. Field of the Invention
The present invention relates to image forming apparatuses, color-misregistration correcting methods, and computer program products.
2. Description of the Related Art
Conventionally, an image forming apparatus for forming a multiple-color image forms a desired image by appropriately overlaying, for instance, CMYK single-color images, or, more specifically, a cyan (C) image, a magenta (M) image, a yellow (Y) image, and a black (K) image, on one another. In some cases, single-color images of an image are misregistered. FIG. 13 is a diagram illustrating an exemplary image, in which misregistration of single-color images has occurred. Each arrow in FIG. 13 indicates a direction, in which misregistration of a single-color image has occurred. Vertical lines, right-down oblique lines, and right-up oblique lines indicate blue, green, and red, respectively. In the example illustrated in FIG. 13, black streaks and a streak of a light color produced by the misregistration of the single-color images are perceptible on the image and hence degrading the image. Furthermore, such misregistration of single-color images can arise a trouble that an image is not reproduced in desired colors or the like.
This misalignment of single-color images (color misregistration) can be caused by a variety of causes, such as variation originated from a manufacture stage of the image forming apparatus per se, a usage condition/handling manner by a user, and/or deterioration with time. It is desired to correct color misregistration because color misregistration degrades an image being formed. This correction is generally performed periodically or irregularly by a user manually or by an automatic correction mechanism mounted on the image forming apparatus.
A conventional color-misregistration correcting method will be described below by way of an example where an inkjet recording apparatus, serving as the image forming apparatus, prints a multiple-color image. FIGS. 14 and 15 are diagrams illustrating the conventional color-misregistration correcting method. In FIGS. 14 and 15, reference symbols Pc, Pm, Py, and Pk indicate a cyan line pattern, a magenta line pattern, a yellow line pattern, and a black line pattern, respectively. According to this method, the patterns Pc, Pm, Py, and Pk are printed on a sheet with ink ejected through ink ejection nozzles of the corresponding colors provided in recording heads of the inkjet recording apparatus. It is assumed that the patterns are printed in a color-misregistered state as illustrated on the left-hand sides of FIGS. 14 and 15. Subsequently, a user visually determines misregistration amounts of the patterns Pc, Pm, Py, and Pk and performs adjustment for the misregistration by using an adjustment mechanism. Alternatively, a colorimeter or the like performs color measurement of the patterns Pc, Pm, Py, and Pk to determine color misregistration amounts based on color differences between the color-measured colors and reference colors so that an automatic correction mechanism automatically corrects misregistration. As a result, such printed patterns having no color misregistration as those illustrated on the right-hand sides of FIGS. 14 and 15 are obtained.
The color difference is an amount represented as, for instance, ΔE below when color difference between a measured color and a reference color is quantified by using the L*a*b* values in the standard CIELAB color space, which is a uniform color space in visual perception.ΔE=√{(L*1−L*2)2+(a*1−a*2)2+(b*1−b*2)2}
Note that L*1, a*1, and b*1 are an L* value, an a* value, and a b* value of the reference color, respectively, while L*2, a*2, and b*2 are an L* value, an a* value, and a b* value of the measured color, respectively.
Examples of techniques related to the conventional color-misregistration correcting method are described in Japanese Patent Application Laid-open No. 2006-192883, Japanese Patent Application Laid-open No. 2006-264270, Japanese Patent Application Laid-open No. 2008-213261, and Japanese Patent Application Laid-open No. 2008-229915.
However, quality of inkjet recording can vary because smear or faint patchy of ink can occur depending on a combination of paper and ink and/or because positions/size of dots can vary depending on a combination of the paper, the ink and an ink ejection capacity. Therefore, a level of image color reproducibility intended by a designer is not always achieved. More specifically, there can be a case in which smear occurs in a considerable amount on some type of paper, causing line patterns to become blurred and making it difficult to measure accurate positions. There can also be a case in which ink ejection per se is not performed normally because of a condition of nozzle surfaces of ink ejection heads or usage environment (e.g., relatively high or low temperature and/or relatively high or low humidity), thereby producing a large number of satellite drops that make the line patterns blurred, or a case in which an ejection amount and/or an ejection velocity is changed, resulting in reproduction in colors that differ from desired colors. Furthermore, there can be a case in which such situations occur in combination. As a result, a problem that color misregistration cannot be detected accurately, causing a misregistration amount having an error to be obtained and making it difficult to perform accurate correction, arises.